US President Donald Trump should use his upcoming trip to South Korea to forge a united front against North Korea's escalating threats, according to a former CIA official.

Jung Pak, former portfolio manager at the CIA's East Asia and Pacific Mission Center, said the president has given the impression of a rift in the alliance by threatening to end the countries' free trade agreement and accusing South Korea of appeasing the North.

"He must convincingly reassure President Moon (Jae-in) and the South Korean public that the US and South Korea are in lockstep on the North Korea issue," she told reporters in a recent meeting. "There must be no daylight between the two leaders. If Trump must absolutely must bring up the (free trade agreement), then do so quietly."

Trump is scheduled to visit South Korea early next month on his first tour of Asia since taking office.

The North Korea issue is expected to dominate the trip as tensions run high over the country's nuclear weapons and ballistic missile programs.

Trump should "highlight that the US and South Korea have and will continue to improve on combined defense capabilities to neutralize a North Korea threat," said Pak, currently the SK-Korea Foundation chair in Korea studies at the Brookings Institution.

"This would also bolster Moon's attempts to tamp down on calls for the return of US tactical nuclear weapons to South Korea or the development of independent nuclear capabilities," she said.

Pak also warned Trump against spending more time in Tokyo, the first leg of his trip, than in Seoul.

"I think that would look really bad, and I think North Korea is going to notice," she said. "If the US president is downplaying the relationship, then why should North Korea pay attention?" (Yonhap)